I think that if someone tried hard enough, these devices can all be hacked. The thing is, would someone care enough to do it? And lastly, isn't it worth the risk if you needed the device to save your life?

If you don't already know it, you might be interested in the story of Barnaby Jack, a black hat hacker who died just before demonstrating that he could hack an ICD. Shortly before his death, he joined this site, attempting to buy used ICDs.